Simplified Tooltip Component

A simplified version of #5. Benefits of this approach:

- Less code.
- All styling in the CSS to make it easier to customise.
- Uses "tooltip" as the JS & CSS file name to better match the ".tooltip" class name.
- Combines the "tooltip" and "tooltip-mega" components as they're functionally very 
similar (removing the need for the "tipsy" library altogether).
- Anchors use HREF to link to the content of their tooltip (like footnotes in Wikipedia) on the title attribute for simple tooltips (so it should degrade nicely if the JS fails).
- Won't reinitialise already initialised components (to potentially work with Turbolinks/PJAX).

Disadvantages:

- No ability to set the position of the tooltip, but that could be handled with something like so:

  HTML:

  ```html
  <a class="tooltip tooltip-left" href="#example">tooltip</a>
  ```

  CSS:

  ```scss
  .tooltip-left {
    .tooltip--outer {
      // etc
    }
    .tooltip--inner {
      // etc
    }
  }
  ```

- No "auto-position" for tooltips as provided by tipsy.